Semiconductor nonvolatile memory element and manufacturing method thereof

ABSTRACT

A semiconductor nonvolatile memory element is used to form a constant current source in a semiconductor integrated circuit device. The semiconductor nonvolatile memory element includes a control gate electrode, a floating gate electrode, source/drain terminals, a thin first gate insulating film, and a second gate insulating film that is thick enough not to be broken down even when a voltage higher than an operating voltage of the semiconductor integrated circuit device is applied thereto, the first and second gate insulating films being formed below the control gate electrode. Thus, provided is a normally on type semiconductor nonvolatile memory element in which a threshold voltage can be regulated through injection of a large amount of charge with respect to the operating voltage from a drain terminal into the floating gate electrode via the second gate insulating film, and injected carriers do not leak in an operating voltage range.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor nonvolatile memoryelement that can change a threshold voltage through an externallyapplied electrical signal and a manufacturing method thereof.

2. Description of the Related Art

An electronic circuit used in electronic equipment is driven by a powersupply such as a battery. When a voltage of the power supply fluctuates,malfunction of the electronic circuit and various abnormal phenomena maybe caused. Thus, it is a typical approach to place a power management ICbetween the electronic circuit and the power supply, which is configuredto regulate the voltage so that a constant voltage is output ormonitoring fluctuations of the power supply, to thereby promote stableoperation. In particular, in a semiconductor integrated circuit devicesuch as a microcomputer or a CPU that is operated at increasingly lowervoltages in recent years, the power management IC has been stronglyrequired to output an accurate constant voltage and to accuratelymonitor the voltage value.

Exemplary power management ICs configured such that a constant voltageis output from a power supply to an electronic circuit include astep-down series regulator as illustrated in FIG. 3.

In this semiconductor integrated circuit device, a power supply voltagethat is applied between a ground terminal 105 and a power supplyterminal 106 is divided by a PMOS output element 104 and a voltagedividing circuit 103 including resistance elements 102. The voltagedivided by the resistance elements 102 is input to a minus inputterminal of an error amplifier 101, and is compared to a certainreference voltage value generated by a reference voltage circuit 100.Depending on a result of the comparison, the error amplifier 101controls an input voltage of the PMOS output element 104 to change asource-drain resistance of the PMOS output element 104. As a result, anoutput terminal 107 has the function of outputting a constant outputvoltage that does not depend on the power supply voltage, but depends onthe reference voltage value of the reference voltage circuit 100 and aresistance divided voltage ratio of the voltage dividing circuit 103.The output voltage is calculated by the following Expression (1):

(Output voltage)=(reference voltage value)×(resistance divided voltageratio of voltage dividing circuit)  (1)

In regulating the output voltage, by changing a resistance value of theresistance element 102 in a method described below, the divided voltageratio of the voltage dividing circuit 103 is changed to set the outputvoltage value at a desired value based on Expression (1). Accordingly,the voltage dividing circuit of the semiconductor integrated circuit isrequired to be processed/corrected for each target output voltage.

Further, a voltage detector as illustrated in FIG. 4 that has thefunction of outputting a signal when the power supply voltage becomes aconstant voltage is also one kind of the power management IC.

In this semiconductor integrated circuit device, the power supplyvoltage that is input from the power supply terminal 108 is converted toa voltage divided by the voltage dividing circuit 103 that includes theresistance elements 102, and the converted voltage is compared to thereference voltage value of the reference voltage circuit 100 by acomparator 108. A voltage signal corresponding to a result of thecomparison is output from the output terminal 107. With this mechanism,a voltage detector is realized that has the function of monitoring thepower supply voltage and outputting, when the voltage becomes equal toor higher than, or, equal to or lower than a certain voltage, a signalfor the purpose of performing appropriate processing.

Also in the example illustrated in FIG. 4, by changing the resistancevalue of the resistance element 102, the divided voltage ratio of thevoltage dividing circuit 103 is changed to set a desired voltagedetection value based on Expression (1). Accordingly, the voltagedividing circuit of the semiconductor integrated circuit device isrequired to be processed/corrected for each target output voltage.

As a resistance element that is used for a voltage dividing circuit of asemiconductor integrated circuit device, a diffused resistor that is amonocrystalline silicon semiconductor substrate implanted withimpurities having a conductivity type opposite to that of thesemiconductor substrate, a resistor formed of polycrystalline siliconimplanted with impurities, or the like is used. In designing the voltagedividing circuit, when a plurality of resistors are used, the resistorsare set so as to have the same length, the same width, and the sameresistivity. Then, the respective resistance elements are equallysubjected to variations in shape in an etching process in which theshape is determined and to variations in impurity implantation.Accordingly, even if the absolute values of the resistance elementsvary, resistance ratios between the resistance elements can bemaintained at a constant value.

FIG. 5 is an illustration of a case in which the resistance elementshaving a certain resistance value based on the same shape and the sameresistivity are used in a voltage dividing circuit. Various resistancevalues are realized through series connection and parallel connection ofunit resistance elements 200 such as resistor groups 201 to 204 in FIG.5. As described above, the unit resistance elements 200 are resistanceelements having the same shape and the same resistivity, and thus, thehigh resistance ratios between the resistor groups each including theunit resistance element (s) having the high resistance ratio can bemaintained with high accuracy.

Further, fuses 301 to 304 of, for example, polycrystalline silicon, areformed in parallel with the resistor groups 201 to 204, respectively, soas to be cut by laser radiation from the outside. Depending on whetheror not the fuses are cut by the laser radiation, a resistance valuebetween a terminal 109 and a terminal 110 can be changed as necessary.Then, a voltage corresponding to a divided voltage ratio to a fixedresistor formed between the terminal 110 and a terminal 111 is outputfrom the terminal 110.

In the voltage dividing circuit as described above that has a highlyaccurate resistance ratio, by cutting the polycrystalline siliconfuse(s) with a laser, a desired divided voltage ratio can be obtainedwith high accuracy, and products having various target output voltagescan be manufactured using the same semiconductor integrated circuitdevice.

A typical method of regulating an output voltage is as illustrated inFIG. 2.

First, an output voltage of a product completed in a semiconductorprocessing factory is measured as it is ((1) in FIG. 2). Then, based ona computational expression or a database prepared in advance dependingon the output voltage, the polycrystalline silicon fuses formed in thevoltage dividing circuit are processed with a laser to trim the outputvoltage ((2) in FIG. 2). Finally, the output voltage of the processedproduct is measured again to see whether or not the product is withinspecification as desired ((3) in FIG. 2). If the product is out ofspecification, the product is not shipped. Other than this, there is anonline trimming method in which the resistors are gradually processedwhile the output voltage is monitored, and the processing is stoppedwhen the output voltage reaches a desired value. The method illustratedin FIG. 2 is called an offline trimming method in contrast with theonline trimming method.

Next, a reference voltage circuit that is used similarly in the circuitsillustrated in FIG. 3 and FIG. 4 is described with reference to FIG. 6Aand FIG. 6B.

A most basic related-art reference voltage circuit includes a depressiontype NMOS transistor 402 and an enhancement type NMOS transistor 401. Asillustrated in FIG. 6A, each of the transistors is formed on a P-typewell region 5 in a semiconductor substrate 1, and includes a gateelectrode 6, a gate insulating film 9, and an N-type source/drain region12. The transistors are different from each other in that, as animpurity region for determining a threshold voltage that is formed underthe gate insulating film 9, an N-channel impurity region 10 is formedwith regard to the depression type NMOS transistor 402 while a P-channelimpurity region 11 is formed with regard to the enhancement type NMOStransistor 401. Further, each of the transistors includes a drainterminal 2 and a source terminal 3 for controlling operation thereof,and a body terminal 4 for fixing a potential of the P-type well region.

By connecting in series the depression type NMOS transistor 402 and theenhancement type NMOS transistor 401 between a power supply terminal 403and a ground terminal 404 as illustrated in FIG. 6B, outputting aconstant current from the depression type NMOS transistor 402 as acurrent source, and inputting the current to the drain terminal 2 of theenhancement type NMOS transistor 401 as a load element, a voltagegenerated at the drain terminal of the enhancement type NMOS transistor401 that is a constant voltage is output to a reference voltage outputterminal 405 (see, for example, Japanese Patent Application Laid-openNo. 2008-198775).

The constant voltage that is output from the reference voltage circuitin this case is as expressed by the following Expression (2):

(Reference voltage circuit constant voltage)=√(Ktd/Kte)×|Vtd|+Vte  (2),

where Vtd and Ktd are a threshold voltage and a transconductance,respectively, of the depression type NMOS transistor, and Vte and Kteare a threshold voltage and a transconductance, respectively, of theenhancement type NMOS transistor.

In other words, variations in output voltage in Expression (1) arisefrom variations in parameters that determine the constant voltage thatis output from the reference voltage circuit. The variations areabsorbed through regulation of the resistance divided voltage ratio ofthe voltage dividing circuit.

SUMMARY OF THE INVENTION

Provided are a semiconductor nonvolatile memory element enablingregulation of a threshold voltage with high accuracy, the semiconductornonvolatile memory element being capable of regulating an output voltagewithout employing any trimming method using laser processing in order toreduce variations in circuit characteristics of a semiconductorintegrated circuit device, and a manufacturing method thereof.

In order to solve the problem described above, the present invention hasthe following configuration.

That is, according to one embodiment of the present invention, there isprovided a semiconductor nonvolatile memory element, including: asemiconductor substrate; a well region of a first conductivity typeformed in the semiconductor substrate; a heavily doped source region anda first heavily doped drain region which are formed so as to be apartfrom each other, and each of which contains high-concentrationimpurities of a second conductivity type; a first gate insulating filmformed on the semiconductor substrate between the heavily doped sourceregion and the first heavily doped drain region so as to be adjacent tothe heavily doped source region; a second gate insulating film formed onthe semiconductor substrate between the heavily doped source region andthe first heavily doped drain region so as to be adjacent to the firstheavily doped drain region; a second heavily doped drain region of thesecond conductivity type formed apart from the heavily doped sourceregion, the second heavily doped drain region including a region underthe second gate insulating film and being formed in a region thatoverlaps the first heavily doped drain region; a first lightly dopeddrain region of the second conductivity type formed apart from theheavily doped source region, the first lightly doped drain regionincluding a region under the first gate insulating film and the regionunder the second gate insulating film and being formed in a region thatoverlaps the first heavily doped drain region and the second heavilydoped drain region; a channel impurity region of the second conductivitytype formed under the first gate insulating film between the heavilydoped source region and the first lightly doped drain region; a floatinggate electrode that is made of polycrystalline silicon containinghigh-concentration impurities and is formed on the first gate insulatingfilm and the second gate insulating film; a third gate insulating filmformed on the floating gate electrode; and a control gate electrode thatis made of polycrystalline silicon containing high-concentrationimpurities, and is formed on the third gate insulating film, in whichthe well region of the first conductivity type includes the heavilydoped source region, the first heavily doped drain region, the secondheavily doped drain region, the first lightly doped drain region, andthe channel impurity region, and is formed to a depth that is largerthan depths of the heavily doped source region, the first heavily dopeddrain region, the second heavily doped drain region, the first lightlydoped drain region, and the channel impurity region.

Further, in order to solve the problem described above, the presentinvention employs the following method.

That is, according to one embodiment of the present invention, there isprovided a method of manufacturing a semiconductor nonvolatile memoryelement, including:

a P-type well region forming step of forming, in a semiconductorsubstrate, a P-type well region containing P-type impurities;

an element isolation insulating film forming step of forming a LOCOSoxide film around the P-type well region;

an N-type heavily doped impurity region forming step of forming, in aregion in which a drain is to be formed, an N-type heavily dopedimpurity region containing N-type impurities;

an N-type lightly doped impurity region forming step of forming a firstN-type lightly doped impurity region at an N-type impurity concentrationthat is lower than an N-type impurity concentration of the N-typeheavily doped impurity region to a depth that is larger than a depth ofthe N-type heavily doped impurity region;

a channel region forming step of forming an N-type impurity region in aregion in which a channel is to be formed in the P-type well region;

a gate insulating film forming step of forming a second gate insulatingfilm in the region in which the drain is to be formed so as to overlapthe N-type heavily doped impurity region, and forming a first gateinsulating film that is thinner than the second gate insulating film inthe region in which the channel is to be formed;

a gate electrode forming step of forming, on the first gate insulatingfilm and on the second gate insulating film, a polycrystalline siliconlayer that contains impurities, forming a third gate insulating film onthe polycrystalline silicon layer, and forming, on the third gateinsulating film, a polycrystalline silicon layer that containsimpurities; and

a source/drain forming step of forming N-type impurity regions in aregion in which a source is to be formed and in the region in which thedrain is to be formed, respectively.

According to the one embodiment of the present invention, thesemiconductor nonvolatile memory element enabling regulation of athreshold voltage with an external electrical signal can be provided,and an output voltage of a semiconductor integrated circuit device canbe regulated with high accuracy and with ease.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow chart for illustrating a method of regulatingan output voltage of a semiconductor integrated circuit device accordingto the present invention.

FIG. 2 is a process flow chart for illustrating a method of regulatingan output voltage of a related-art semiconductor integrated circuitdevice.

FIG. 3 is a schematic circuit diagram of a step-down series regulator ofthe related-art semiconductor integrated circuit device.

FIG. 4 is a schematic circuit diagram of a voltage detector of therelated-art semiconductor integrated circuit device.

FIG. 5 is an illustration of an exemplary voltage dividing circuit inwhich related-art resistance elements are combined.

FIG. 6A is a schematic sectional view for illustrating a related-artreference voltage circuit.

FIG. 6B is an illustration of an exemplary related-art reference voltagecircuit.

FIG. 7A is a schematic sectional view for illustrating a referencevoltage circuit according to the present invention.

FIG. 7B is an illustration of an exemplary reference voltage circuitaccording to the present invention.

FIG. 8 is a schematic circuit diagram of a step-down series regulator ofthe semiconductor integrated circuit device according to the presentinvention.

FIG. 9 is a schematic circuit diagram of a voltage detector of thesemiconductor integrated circuit device according to the presentinvention.

FIG. 10 is a schematic sectional view for illustrating a semiconductornonvolatile memory element according to a first embodiment of thepresent invention.

FIG. 11 is a schematic sectional view for illustrating a semiconductornonvolatile memory element according to a second embodiment of thepresent invention.

FIG. 12 is a schematic sectional view for illustrating a semiconductornonvolatile memory element according to a third embodiment of thepresent invention.

FIG. 13 is a schematic sectional view for illustrating a semiconductornonvolatile memory element according to a fourth embodiment of thepresent invention.

FIG. 14A, FIG. 14B, and FIG. 14C are schematic sectional views forillustrating a semiconductor nonvolatile memory element according to afifth embodiment of the present invention.

FIG. 15A, FIG. 15B, and FIG. 15C are schematic sectional views forillustrating a semiconductor nonvolatile memory element according to asixth embodiment of the present invention.

FIG. 16A, FIG. 16B, and FIG. 16C are schematic sectional views forillustrating a semiconductor nonvolatile memory element according to aseventh embodiment of the present invention.

FIG. 17A, FIG. 17B, and FIG. 17C are schematic sectional views forillustrating a semiconductor nonvolatile memory element according to aneighth embodiment of the present invention.

FIG. 18 is an equivalent circuit diagram of capacitances of gateinsulating films when seen from a drain terminal according to thepresent invention.

FIG. 19A and FIG. 19B are graphs for showing electrical characteristicswhen the present invention is applied to a step-down series regulator.

FIG. 20A, FIG. 20B, FIG. 20C, and FIG. 20D are sectional views forillustrating process steps for manufacturing the semiconductornonvolatile memory element according to the first embodiment of thepresent invention.

FIG. 21E, FIG. 21F, FIG. 21G, and FIG. 21H are sectional views forillustrating process steps for manufacturing the semiconductornonvolatile memory element according to the first embodiment of thepresent invention, continued from FIG. 20D.

FIG. 22A, FIG. 22B, FIG. 22C, and FIG. 22D are sectional views forillustrating process steps for manufacturing the semiconductornonvolatile memory element according to the second embodiment of thepresent invention.

FIG. 23A, FIG. 23B, FIG. 23C, and FIG. 23D are sectional views forillustrating process steps for manufacturing the semiconductornonvolatile memory element according to the third embodiment of thepresent invention.

FIG. 24E and FIG. 24F are sectional views for illustrating process stepsfor manufacturing the semiconductor nonvolatile memory element accordingto the third embodiment of the present invention, continued from FIG.23D.

FIG. 25A, FIG. 25B, FIG. 25C, and FIG. 25D are sectional views forillustrating process steps for manufacturing the semiconductornonvolatile memory element according to the fourth embodiment of thepresent invention.

FIG. 26A and FIG. 26B are sectional views for illustrating a secondgroup of process steps for manufacturing a first gate insulating filmand a second gate insulating film of the semiconductor nonvolatilememory element according to the present invention.

FIG. 27A, FIG. 27B, and FIG. 27C are sectional views for illustrating athird group of process steps for manufacturing the first gate insulatingfilm and the second gate insulating film of the semiconductornonvolatile memory element according to the present invention.

FIG. 28A, FIG. 28B, and FIG. 28C are sectional views for illustrating afourth group of process steps for manufacturing the first gateinsulating film and the second gate insulating film of the semiconductornonvolatile memory element according to the present invention.

FIG. 29A, FIG. 29B, FIG. 29C, and FIG. 29D are sectional views forillustrating a fifth group of process steps for manufacturing the firstgate insulating film and the second gate insulating film of thesemiconductor nonvolatile memory element according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Now, embodiments of the present invention are described with referenceto the attached drawings.

First, an embodiment in which the present invention is applied to therelated-art semiconductor integrated circuit device described above withreference to FIG. 3 and FIG. 4 is described with reference to FIG. 8 andFIG. 9. As illustrated in FIG. 8 and FIG. 9, an input terminal forregulation 112 for inputting a voltage/current applied from the outsideis added to the reference voltage circuit 100. A specific element in thereference voltage circuit is replaced by a semiconductor nonvolatilememory element. The semiconductor nonvolatile memory element can changea threshold voltage depending on the voltage/current input to the inputterminal for regulation 112 from the outside.

A method of regulating an output voltage is now described with referenceto FIG. 1.

First, an output voltage of a product completed in a semiconductorprocessing factory is measured as it is (a process step of (1) in FIG.1).

Then, a voltage/current is applied to the semiconductor nonvolatilememory element in the reference voltage circuit via the input terminalfor regulation to change the threshold voltage of the semiconductornonvolatile memory element (a process step of (2) in FIG. 1). In thesemiconductor integrated circuit device as illustrated in FIG. 8 or FIG.9, when the reference voltage value that is output from the referencevoltage circuit changes, in accordance with Expression (1), the outputvoltage is also changed in proportion thereto, and thus, the amount ofthe voltage/current applied to the input terminal for regulation and anamount of the output voltage are directly proportional to each other.

After that, the output voltage is measured. If the output voltage is outof tolerance required for the product, the process returns to theprocess step of (2) in FIG. 1 to start again application of avoltage/current to the semiconductor nonvolatile memory element. In thiscase, a method in which the reference voltage value of the referencevoltage circuit is set in advance so that an initial output voltagevalue is out of specification and the voltage/current applied to thesemiconductor nonvolatile memory element is gradually changed in onedirection, i.e., in a +direction or in a −direction to fit the productinto specification is an easy way of regulation, and thus, is preferred.

The process step of (2) and a process step of (3) in FIG. 1 arerepeated, and, when the output voltage value is within specification,the series of processing ends (a process step of (4) in FIG. 1). Theprocess step of (2) and the process step of (3) in FIG. 1 can be, inreality, performed not intermittently but electrically continuously, andthus, by writing a program software and exerting automatic control, ittakes only a short time to fit an out-of-specification product intospecification.

By adopting such a method, the related-art three-step process from theprocess step of (1) to the process step of (3) in FIG. 2 that cannot beredone can be completed in single electrical processing, which cansimplify the method of regulating the output voltage and can reduce to alarge extent time necessary for completing the product. Further, themethod regulates the output voltage through online trimming during whichthe output voltage is monitored, and thus, faulty products out ofspecification are inhibited, and improvement in yield can be expected.

Further, influence of high heat (temperature coefficient andrecrystallization of the resistors) such as in a related-art onlinetrimming through processing of resistors with a laser can be eliminated,and thus, it is not necessary to fear an error in and reregulation ofthe output voltage, and a stable output voltage can be maintained.

Further, this regulating method is electrical processing that isperformed irrespective of a form of the product (wafer or package), andthus, even if the form of the product changes and the characteristicsfluctuate accordingly, electrical reregulation through a terminal can beperformed. For example, when an output voltage of a product in a waferstate is regulated, and then, after the product is packaged, the outputvoltage is changed under the influence of thermal hysteresis, resinstress, or the like and the product goes out of specification, theproduct in a package state can be reregulated to be fit intospecification. Alternatively, the output voltage may be regulated onlyin a final form. By omitting measurement/processing in the wafer state,the frequency of the test can be lowered and the number of the processsteps can be reduced.

Further, in addition to the lowered frequency of the test describedabove, the need for the laser trimming step is eliminated, and thus, theeffect of inhibiting investment in equipment such as the measurementapparatus and the laser apparatus is also great.

Further, the voltage dividing circuit 103 including the resistanceelements 102 illustrated in FIG. 8 or FIG. 9 is not required to behighly accurate. Even if the accuracy of the voltage dividing circuit103 is low, the method according to the present invention enables theoutput voltage value to be fit into specification, and thus, preparing aplurality of uniform resistance elements and consideration of a patternlayout thereof as in the related art are not necessary, and the fuseelements are also not necessary. Accordingly, there is an advantage inthat the chip size and a layout load are expected to be reduced.

Next, a reference voltage circuit for carrying out the present inventionis described with reference to FIG. 7A and FIG. 7B. As illustrated inFIG. 7B, in the reference voltage circuit, the depression type NMOStransistor 402 and the enhancement type NMOS transistor 401 areconnected in series between an input terminal for regulation 406 and theground terminal 404. A constant current is output from the depressiontype NMOS transistor 402 serving as a current source. A voltagegenerated at the drain terminal of the enhancement type NMOS transistor401 serving as a load element is output as a constant voltage to thereference voltage output terminal 405.

However, in this case, as illustrated in FIG. 7A, the depression typeNMOS transistor 402 used in the present invention has a structure inwhich polycrystalline silicon gate electrodes are stacked, with an upperlayer being a control gate electrode 8 for controlling the voltage, anda lower layer being a floating gate electrode 7 for injecting andaccumulating charge.

In the exemplary circuit structure illustrated in FIG. 7B, when avoltage at the input terminal for regulation 406 rises, because avoltage between the reference voltage output terminal 405 and the groundterminal 404 is fixed at a certain value at all times, the voltage riseis borne between the input terminal for regulation 406 and the referencevoltage output terminal 405. Accordingly, as a voltage applied to theinput terminal for regulation 406 rises, a drain-source voltage of thedepression type NMOS transistor 402 rises. Carriers with charges, inthis case, holes are injected into, via a gate insulating film, thefloating gate electrode 7 at a lower potential by a method to bedescribed below, thereby being capable of charging the floating gateelectrode to a positive side. This is equivalent to a lowered thresholdvoltage of the depression type NMOS transistor, when seen from a controlgate electrode side. Under the influence of this, the amount of currentof the depression type NMOS transistor increases, and the potential ofthe reference voltage output terminal 405 increases accordingly.

When the reference voltage value of the reference voltage circuitincreases, in accordance with Expression (1), the output voltage of thestep-down series regulator illustrated in FIG. 8 rises. Specifically, bycontrolling the voltage of the reference voltage circuit input terminal,the output voltage of the step-down series regulator can be arbitrarilychanged. Note that, in this case, the input terminal for regulation 112corresponds to the input terminal for regulation 406 illustrated in FIG.7A and FIG. 7B.

In this case, the threshold voltage of the semiconductor nonvolatilememory element is changed to a negative direction by voltage regulationvia the input terminal for regulation, and thus, in accordance withExpression (2), the value of Vtd that is originally negative is changedfurther to the negative side, |Vtd| as an absolute value thereofincreases, and the reference voltage that is output from the referencevoltage circuit is changed to a higher direction. Then, the outputvoltage of the step-down series regulator is changed in a higherdirection accordingly. Thus, when the output voltage of the step-downseries regulator according to the present invention is designed so as tohave a lower value than that of required specifications beforeregulation by the input terminal for regulation, through regulation ofthe output voltage by the input terminal for regulation, a wide range ofrequired specifications on the output voltage can be accommodated.

Further, according to this method, the output voltage can be fit into apredetermined target voltage value not via a laser trimming step butonly through electrical control with high accuracy.

A specific example is described with reference to FIG. 19A and FIG. 19B.In graphs of FIG. 19A and FIG. 19B, the horizontal axis denotes avoltage that is input to the input terminal for regulation 406 of thereference voltage circuit as illustrated in FIG. 7A and FIG. 7B and thevertical axis denotes a voltage that is output from the output terminal107 of the step-down series regulator as illustrated in FIG. 8 includingthe reference voltage circuit. FIG. 19A shows output voltagecharacteristics before the regulation by the input terminal forregulation, and FIG. 19B shows output voltage characteristics after theregulation.

First, before the regulation of the input, as shown in FIG. 19A, untilthe input voltage reaches a point (a) (range in which the referencevoltage circuit normally operates), the output voltage increases as theinput voltage to the reference voltage circuit increases. When theoutput voltage reaches the voltage calculated by Expression (1), theoutput voltage remains constant until a point (b) is reached. Up to thispoint, the electrical characteristics are completely the same as thoseof the related-art step-down series regulator.

After that, when the point (b) is reached at which the input voltagebecomes high enough for the carriers to be injected into the floatinggate electrode of the semiconductor nonvolatile memory element,injection of the carriers into the semiconductor nonvolatile memoryelement begins. At the same time, the threshold voltage of thesemiconductor nonvolatile memory element changes. Accordingly, inaccordance with the amount of the injected carriers, the output voltagebegins to increase again. When application of higher input voltage isstopped when a point (c) is reached at which the output voltage has adesired value, the carrier injection into the semiconductor nonvolatilememory element is stopped, and the carriers are stored in the floatinggate electrode. The electrical characteristics after the above-mentionedoperations end are as shown in FIG. 19B.

In other words, the threshold voltage of the semiconductor nonvolatilememory element changes in accordance with the amount of the carriersinjected into the semiconductor nonvolatile memory element, and thus,|Vtd| increases in accordance with Expression (2), and the constantoutput voltage in accordance with the reference voltage circuit constantvoltage and Expression (1) is also changed to the high output voltage atthe point (c). When a voltage equal to or higher than that at the point(b) is applied to the input terminal for regulation, carrier injectioninto the semiconductor nonvolatile memory element begins again and theoutput voltage begins to increase again.

Attention needs to be given to, first, a point that, in order to enablethe output voltage regulation as shown in FIG. 19A, the semiconductornonvolatile memory element needs to be designed so as to have asufficiently high drain breakdown voltage so that the element may not bebroken down even when the input voltage reaches the value of the point(c).

A second point to which attention needs to be given is that, after theoutput voltage is regulated to have the desired value, a maximum valueof the input voltage when the product is actually used needs to besufficiently lower than that at the point (b) so that the output voltagemay not be changed due to the carrier reinjection while the product isused. In other words, it is necessary that a semiconductor integratedcircuit device to which the present invention is applied has such aproduct specification that an operating voltage thereof is equal to orlower than the voltage at the point (b) without fail. Accordingly,semiconductor nonvolatile memory elements having characteristicssuitable for respective product specifications of semiconductorintegrated circuit devices are prepared in advance.

Similarly, by realizing the reference voltage circuit in a voltagedetection circuit illustrated in FIG. 4 using a similar circuit, theoutput voltages of the respective semiconductor integrated circuitdevices can be similarly controlled and set through voltage control ofthe input terminal for regulation connected to the reference voltagecircuit.

Further, with regard to the reference voltage circuit, it goes withoutsaying that the present invention can be applied to any structure of acircuit insofar as the circuit basically operates with a combination ofan element serving as a current source and an element serving as a loadas described above.

Further, the semiconductor nonvolatile memory element described hereinis meant to refer to any kind of element whose threshold voltage can beshifted through carrier injection such as charge injection into thefloating gate electrode as hot carrier injection, carrier injectionusing an FN tunnel current via a gate insulating film, or trapping ofcarriers in a level existing in an insulating film.

Next, the semiconductor nonvolatile memory element to which the presentinvention is applied is described in detail.

FIG. 10 is a sectional view for illustrating a semiconductor nonvolatilememory element according to the first embodiment of the presentinvention. The element illustrated in FIG. 10 is formed in a P-type wellregion 5 containing boron as impurities at a concentration of from about7×10¹⁵/cm³ to about 7×10¹⁶/cm³ that is formed on a semiconductorsubstrate 1. A depression type NMOS transistor including N-typesource/drain regions 12, an N-type channel impurity region 10, gateinsulating films 9, 14, and 15, and gate electrodes 7 and 8 is formed ina region surrounded by a LOCOS oxide film 13 used for element isolationand having a thickness of several thousands of angstromes to 2 μm.

Features are that the gate electrode has a structure in which thefloating gate electrode 7 and a control gate electrode 8 that are formedof polycrystalline silicon are stacked, that the control gate electrodeis connected to a source terminal 3 of the depression type NMOStransistor via metal wiring or the like (not shown), and that thefloating gate electrode 7 is surrounded by the first gate insulatingfilm 9, the second gate insulating film 14, and the third gateinsulating film 15 and is in a state of having no electrical connection.

Carriers such as electrons or holes are injected into the floating gateelectrode 7 from a drain terminal 2 via the second gate insulating film14. When positive or negative carriers are injected into the floatinggate electrode 7, similarly to a case of fixed charge existing between agate electrode and a channel region in an ordinary MOSFET, the thresholdvoltage of the depression type NMOS transistor changes in accordancewith the injected amount.

Through holding the injected carriers under such structural conditionsthat the carriers do not escape with heat or electrical stress in normaloperation, the threshold voltage of the depression type NMOS transistorcan be regulated to and maintained at a desired value.

The threshold voltage when carriers are not injected into the floatinggate electrode 7 is set to be a certain negative value in accordancewith the amount of impurities in the N-type channel impurity region 10so that the depression type NMOS transistor is in a normally on state inwhich current flows even when a gate-source voltage is 0 V, whenever avoltage is applied between the drain and the source.

The N-type source/drain regions 12 function as source/drain terminals bylowering resistance thereof under a state in which N-type impuritiessuch as As or P of 1×10²⁰/cm³ or more are injected thereinto. The drainterminal side further includes an N-type heavily doped impurity region17 containing N-type impurities such as As or P of 5×10¹⁸/cm³ or moreand a first N-type lightly doped impurity region 18 containing N-typeimpurities such as As or P of 1×10¹⁷/cm³ or more and 1×10¹⁸/cm³ or lessbetween the drain terminal side and the channel impurity region 10.

The first N-type lightly doped impurity region 18 plays a role inrelaxing an electric field through extension of a depletion layer on thedrain side when a drain voltage is applied and in realizing thereby ahigher breakdown voltage. The N-type heavily doped impurity region 17 isused as a lower electrode when carriers are injected into the floatinggate electrode 7.

A first reason why the impurity concentration of the N-type heavilydoped impurity region 17 is 5×10¹⁸/cm³ or more is to prevent injectionefficiency from being lowered due to a depleted surface of the N-typeheavily doped impurity region 17 to which a positive voltage is appliedwhen holes are injected as carriers into the floating gate electrode 7and due to a relaxed electric field between the floating gate electrode7 and the N-type heavily doped impurity region 17.

A second reason is to prevent a depletion layer between the drain and awell that extends from the N-type lightly doped impurity region 18 tothe drain side when, similarly, a high voltage is applied to the N-typeheavily doped impurity region 17 for the purpose of injecting holes ascarriers into the floating gate electrode 7 and to thereby preventcarrier injection efficiency from being lowered.

Incidentally, the N-type heavily doped impurity region 17 and thefloating gate electrode 7 have an overlapping portion 16. The secondgate insulating film 14 that exists in the overlapping portion 16 has athickness that is different from that of the gate insulating film 9formed on the N-type channel impurity region 10. In general, a gateinsulating film is set to have a predetermined thickness in accordancewith an operating voltage of a semiconductor integrated circuit deviceincluding a MOSFET that includes the gate insulating film from theviewpoint of reliability for a long time. However, the second gateinsulating film 14 of the semiconductor nonvolatile memory elementaccording to this embodiment adopts a thickness that is determined so asto be suitable for carrier injection into the floating gate electrode 7and that is larger than the above-mentioned thickness determined by thereliability for a long time in accordance with the operating voltage forthe purpose of preventing charge from escaping within an operatingvoltage range.

Accordingly, according to this embodiment, the second gate insulatingfilm 14 has a thickness that is larger than that of the gate insulatingfilm 9 formed on the N-type channel impurity region 10.

Another feature of the semiconductor nonvolatile memory elementaccording to the present invention is that regulation specialized forcharacteristics of a semiconductor nonvolatile memory element andsemiconductor integrated circuit device including the same is performedin an analog way, and the regulation is performed through replacement ofan element that is a part of a related-art semiconductor nonvolatilememory element. Accordingly, formation of a memory array for the purposeof storing information is not assumed, and a structure such as a selectgate for identifying an address that is necessary when a memory array isformed is not required.

Next, electrical operation according to the present invention isdescribed in detail.

For example, a potential of the floating gate electrode 7 changes to anegative potential when electrons having negative charge are injectedthereinto. In that case, in response to the negative charge, positivecharge is induced in the channel region, or, electrons in the N-typechannel impurity region 10 reduce, and the threshold voltage of theN-channel type MOSFET changes to the positive side.

On the other hand, when holes having positive charge are injected intothe floating gate electrode 7, the potential of the floating gateelectrode 7 is shifted to the positive side and an electronconcentration in the N-type channel impurity region 10 increases. As aresult, the threshold voltage of the N-channel type MOSFET changes tothe negative side.

According to the present invention, due to the existence of the N-typechannel impurity region 10, the threshold voltage of the depression typeNMOS transistor has a negative value under a state in which no carrieris injected, and thus, through application of a positive potential tothe floating gate electrode 7 and injection of holes as carriers fromthe N-type heavily doped impurity region 17 side, the negative thresholdvoltage is changed further to the negative side, and the thresholdvoltage is controlled with high accuracy.

An ordinary semiconductor nonvolatile memory element is under binarycontrol with threshold voltages, for example, one is set higher than 0 Vand the other is set lower than 0 V. A plurality of such elements iscombined to be used for holding necessary information in a digital way.The present invention is different from the related-art method in thatonly one element is used, information is determined in an analog way bythe amount of carriers in the floating gate electrode, and theinformation is kept held.

According to the present invention, making full use of such a functionof changing the threshold voltage and nonvolatility, carriers areinjected into the semiconductor nonvolatile memory element in advancebefore shipment to a customer to regulate the threshold voltage of thesemiconductor nonvolatile memory element, thereby regulating circuitcharacteristics of a semiconductor integrated circuit device includingthe semiconductor nonvolatile memory element to have a desired value.After that, no carriers are put in/out during usage by the customer, andreliability at a high level for repeated erasing and writing is notnecessary for the second gate insulating film 14.

According to the present invention, carriers are injected into thefloating gate electrode 7 as follows. First, the amount of impurities inthe N-type channel impurity region 10 is set such that, under a state inwhich no carriers are injected, the threshold voltage is set to be of anegative value that is higher than (on the positive side with respectto) a final target value.

Then, in a step of testing the semiconductor integrated circuit deviceafter a semiconductor manufacturing process, the source potential andthe control gate electrode potential are set to be the same lowpotential while the drain potential is changed to a higher potential onthe positive side. In this state, the floating gate potential is of avalue between the drain potential and the source potential/control gateelectrode potential, in accordance with a capacitance ratio that isdetermined by thicknesses of the first gate insulating film 9, thesecond gate insulating film 14, and the third gate insulating film 15and by sizes of the control gate electrode 8 and the floating gateelectrode 7. Through regulation of the sizes and the thicknessesdescribed above, the value is set to be a low value close to the sourcepotential/control gate electrode potential so that most of thedrain-source voltage is applied to the second gate insulating film 14.

In this depression type NMOS transistor, even if the potential of thecontrol gate electrode 8 is 0 V, the threshold voltage is of a negativevalue, and thus, as the drain potential rises, a drain current flows.After the current characteristics goes out of an non-saturated regioninto a saturated region, the current is stabilized at a constantsaturation current value that does not depend on the drain potential. Apotential at an end of the depletion layer generated on the drain sidein the N-type channel impurity region 10 is fixed to a low constantvalue (pinch-off voltage) determined by the gate-source voltage and thethreshold voltage, and thus, a voltage that is a difference between thedrain-source voltage and the pinch-off voltage is applied to thedepletion layer generated in the first N-type lightly doped impurityregion 18. The depletion layer has an impurity concentration that islower than the sufficiently high impurity concentration in the N-typeheavily doped impurity region 17, and thus, with regard to the potentialof the N-type heavily doped impurity region 17, the drain potentialapplied to the drain terminal 2 is applied as it is to the N-typeheavily doped impurity region 17, and it is easy to control a potentialdifference between the N-type heavily doped impurity region 17 and thecontrol gate electrode 8.

At this time, by arbitrarily setting the impurity concentration and alength in a plane direction of the first N-type lightly doped impurityregion 18, the amount of extension of the depletion layer generated inthe N-type lightly doped impurity region 18 can be controlled, and anupper limit of the applied drain voltage due to avalanche breakdown canbe raised. This enables setting a potential applied to the N-typeheavily doped impurity region 17 to have a large value, and thus, apotential for injecting carriers can be secured even if the second gateinsulating film 14 is set to have a large thickness.

For example, by setting the first N-type lightly doped impurity region18 to have an impurity concentration of from 1×10¹⁷/cm³ to 1×10¹⁸/cm³and setting a length from the channel region 10 to the N-type heavilydoped impurity region 17 in the plane direction to be 1.5 μm or more,the drain breakdown voltage can be 20 V or higher, and a carrierinjection voltage of 20 V or higher can be secured.

In this case, when the second gate insulating film 14 has a thicknessof, for example, 400 Å, tunneling can be induced by applying a voltageof about 20 V or higher. Thus, in the drain structure described above,by setting the drain voltage to be 20 V or higher, injection of holeshaving positive charge based on the tunneling is realized in theoverlapping portion 16 of the N-type heavily doped impurity region 17and the floating gate electrode 7. On the other hand, a voltage appliedto the N-type channel impurity region 10 is equal to or lower than thepinch-off voltage described above, and thus, insofar as an insulatingfilm thickness in accordance with the pinch-off voltage is secured,tunneling is not induced in the first gate insulating film 9 between theN-type channel impurity region 10 and the floating gate electrode 7.

As described above, it is preferred that the drain voltage applied inthe testing step be sufficiently higher than the operating voltage ofthe semiconductor integrated circuit device including this semiconductornonvolatile memory element. This can prevent carriers from beinginjected into the floating gate electrode 7 while a power supply voltagefluctuates within the range of the operating voltage of thesemiconductor integrated circuit device, to thereby inhibit fluctuationsin threshold voltage of the semiconductor nonvolatile memory element andchange in circuit characteristics of the semiconductor integratedcircuit device that is caused due to the fluctuations. For example, inthe example described above, it is desired that the operating voltage ofthe semiconductor integrated circuit device be 10 V or lower. In thisway, for the purpose of securing a sufficient potential differencebetween the operating voltage and the carrier injection voltage (in theexample described above, 20 V−10 V=10 V), it is necessary to set thethickness of the second gate insulating film 14 and the conditions onthe first N-type lightly doped impurity region 18.

The amount of carriers to be stored in the floating gate electrode 7 canbe determined through gradual increase in drain voltage as illustratedin FIG. 19A and FIG. 19B. Alternately, the amount of charge of holes tobe stored in the floating gate electrode 7 can also be controlled by aproduct of the value of the drain voltage and a time period ofapplication thereof. As described above, the threshold voltage of thedepression type NMOS transistor is shifted further to the negative sidein accordance with the amount of stored charge of the holes, and thus,it is possible to apply a high constant drain voltage until a desiredthreshold voltage is reached, to thereby regulate, with high accuracy,the threshold voltage of the N-channel type MOSFET with the time periodof application thereof while the output voltage is being monitored.

FIG. 11 is a sectional view for illustrating a semiconductor nonvolatilememory element according to a second embodiment of the presentinvention. In the semiconductor nonvolatile memory element illustratedin FIG. 11, in addition to the structure illustrated in FIG. 10, asecond N-type lightly doped impurity region 19 containing As or P havingan impurity concentration of from about 2×10¹⁶/cm³ to about 2×10¹⁷/cm³is added under the first N-type lightly doped impurity region 18. In thecase illustrated in FIG. 10, depending on the conditions on the firstN-type lightly doped impurity region 18, it is easy to set the drainbreakdown voltage to be as high as about 30 V.

However, with regard to the depletion layer in the first N-type lightlydoped impurity region 18 and in the P-type well region 5 thereunder,extension of the depletion layer to the first N-type lightly dopedimpurity region 18 side is limited, and it is difficult to set thebreakdown voltage to be higher than 30 V. Accordingly, by adding thesecond N-type lightly doped impurity region 19 as illustrated in FIG. 11to secure extension of the depletion layer corresponding to a diffusiondepth thereof, a drain breakdown voltage that is higher than 30 V can beattained. This is effective in accommodating a semiconductor integratedcircuit device having a higher operating voltage and in securing alarger margin between the operating voltage and a tunneling voltage.

FIG. 12 is a sectional view for illustrating a semiconductor nonvolatilememory element according to a third embodiment of the present invention.In the semiconductor nonvolatile memory element illustrated in FIG. 12,an oxide film 13 having a thickness that is larger than those of thefirst gate insulating film 9 and the second gate insulating film 14 isformed between the floating gate electrode 7 and the first N-typelightly doped impurity region 18. Such a structure can relax a highelectric field between the floating gate electrode 7 having a lowpotential and the first N-type lightly doped impurity region 18 that isinduced when the drain voltage is increased, and the drain breakdownvoltage can be set to be as high as about 60 V.

The thick oxide film 13 can have an arbitrary thickness depending on theextent of the necessary relaxation of the electric field. When the drainbreakdown voltage is higher than 30 V, it is preferred that thethickness be 1,000 Å or more. Further, by forming the thick oxide film13 simultaneously with a LOCOS oxide film in the element isolationregion, increase in the number of process steps can be prevented.

FIG. 13 is a sectional view for illustrating a semiconductor nonvolatilememory element according to a fourth embodiment of the presentinvention. In the semiconductor nonvolatile memory element illustratedin FIG. 13, the second N-type lightly doped impurity region 19illustrated in FIG. 12 is extended to a direction to the source terminal3 side to an extent so as to overlap the N-type channel impurity region10. In addition, a P-type lightly doped impurity region 20 having animpurity concentration that is higher than that of the second N-typelightly doped impurity region 19 is formed so as to surround the sourceterminal 3.

The P-type lightly doped impurity region 20 is formed in a region thatis close to the first gate insulating film 9 and that does not exceedthe thick oxide film 13, and contains B or BF₂ having an impurityconcentration of from about 2×10¹⁶/cm³ to about 2×10¹⁷/cm³, which ishigher than that of the second N-type lightly doped impurity region 19.The structure in which the P-type lightly doped impurity region 20 hasan impurity concentration that is higher than that of the second N-typelightly doped impurity region 19 enables further extension to the drainside of the depletion layer on the channel side and on the drain sidethat is generated when the drain voltage is increased, which iseffective when a drain breakdown voltage of 60 V or higher is requiredto be obtained.

In the first to fourth embodiments described above, as illustrated inFIG. 10 to FIG. 13, the floating gate electrode 7 and the control gateelectrode 8 are formed by stacked polycrystalline silicon layers. Such amethod using two polycrystalline silicon layers can inhibit increase inarea of the gate electrode to facilitate lower costs, but the number ofprocess steps increases and the processing is complicated. This leadsto, for example, difficult selection of conditions on dry etching whenthe floating gate electrode 7, the control gate electrode 8, and thethird gate insulating film 15 therebetween are collectively processedand deteriorated flatness due to etching resistance of a resist to be amask, a stringer of polycrystalline silicon formed at a portion having alevel difference, or a high-aspect-ratio gate electrode structure.

FIG. 14A to FIG. 17C are illustrations of methods of realizing asemiconductor nonvolatile memory element using only one polycrystallinesilicon layer for the purpose of overcoming the complication, andcorrespond to the structures illustrated in FIG. 10 to FIG. 13,respectively.

First, FIG. 14A, FIG. 14B, and FIG. 14C are illustrations of a fifthembodiment of the present invention in which the two-layer gateelectrode structure of polycrystalline silicon illustrated in FIG. 10 ischanged to a single-layer structure.

FIG. 14A is a plan view, and FIG. 14B and FIG. 14C are sectional viewstaken along the lines A-A′ and B-B′, respectively, of FIG. 14A. Thepolycrystalline silicon structure including two stacked layersillustrated in FIG. 10 is changed to a single-layer structure includingonly the floating gate electrode 7. As illustrated in FIG. 14B, thecontrol gate electrode 8 and the third gate insulating film 15 are notformed on the floating gate electrode 7. Instead, as illustrated in FIG.14A, the floating gate electrode 7 extends beyond the channel region soas to overlap the control gate electrode 8 that uses an impuritydiffusion region in the semiconductor substrate 1. The control gateelectrode 8 that uses the impurity diffusion region in the semiconductorsubstrate 1 includes a potential output portion 6. The control gateelectrode 8 may use the impurities and a structure and a process stepof, for example, the N-type heavily doped impurity region 17 in common,and the N-type high-concentration impurities in the source/drain regions12 may also be used in the potential output portion 6.

Further, the third gate insulating film 15 between the floating gateelectrode 7 and the control gate electrode 8 as used in the structureillustrated in FIG. 10 is formed of an oxide film on a surface of thesemiconductor substrate 1 that is formed between the floating gateelectrode 7 and the control gate electrode 8 serving as the impuritydiffusion region in the semiconductor substrate 1. In this case, anoxide film formed simultaneously with the first gate insulating film 9formed outside the channel region 10 is used.

The structure illustrated in FIG. 14A, FIG. 14B, and FIG. 14C requirestwo occupation areas for the control gate electrode 8 and the floatinggate electrode 7 in the semiconductor integrated circuit device, andthus, the occupation area in the chip increases and cost also increasesaccordingly. However, the present invention is not applied to a memoryarray in which a large number of semiconductor nonvolatile memoryelements are arrayed in a semiconductor integrated circuit device. Thus,the increase in occupied area is not to a large extent, and the increasein costs is almost no problem in the semiconductor integrated circuitdevice. On the other hand, as described above, the present invention hasa strong point that, through elimination of the complication and thedifficulty in manufacturing process, the effects of stabilized qualityand a reduced number of process steps can be enjoyed.

Further, in the structure illustrated in FIG. 14A, FIG. 14B, and FIG.14C, when a circuit is adopted in which the gate potential and thesource potential, and the potential of the P-type well region that areused are a common low potential realized through connection using metalwiring or the like, the impurities in the control gate electrode 8illustrated in FIG. 14C may be P-type high-concentration impurities, andfurther, the control gate electrode 8 may be the P-type well region 5 asit is.

The reason is that the semiconductor nonvolatile memory elementaccording to the present invention is of a normally on type in which,due to the existence of the N-type channel impurity region 10, a currentflows in accordance with the drain voltage even when the gate-sourcevoltage is 0 V. Accordingly, when the P-type well region 5 is connectedto the source terminal 3 using metal wiring or the like (not shown)located somewhere, the potential relationship is the same even if theimpurities diffused in the control gate electrode 8 are of the P type.

FIG. 15A, FIG. 15B, and FIG. 15C are illustrations of a sixth embodimentof the present invention in which the two-layer gate electrode structureof polycrystalline silicon illustrated in FIG. 11 is changed to asingle-layer structure, which has an effect that is similar to that ofthe fifth embodiment described with reference to FIG. 14A, FIG. 14B, andFIG. 14C. FIG. 15A is a plan view, and FIG. 15B and FIG. 15C aresectional views taken along the lines A-A′ and B-B′, respectively ofFIG. 15A.

Further, FIG. 16A, FIG. 16B, and FIG. 16C are illustrations of a seventhembodiment of the present invention in which the two-layer gateelectrode structure of polycrystalline silicon illustrated in FIG. 12 ischanged to a single-layer structure, which has an effect that is similarto that of the fifth embodiment described with reference to FIG. 14A,FIG. 14B, and FIG. 14C. FIG. 16A is a plan view, and FIG. 16B and FIG.16C are sectional views taken along the lines A-A′ and B-B′,respectively of FIG. 16A.

Further, FIG. 17A, FIG. 17B, and FIG. 17C are illustrations of an eighthembodiment of the present invention in which the two-layer gateelectrode structure of polycrystalline silicon illustrated in FIG. 13 ischanged to a single-layer structure, which has an effect that is similarto that of the fifth embodiment described with reference to FIG. 14A,FIG. 14B, and FIG. 14C. FIG. 17A is a plan view, and FIG. 17B and FIG.17C are sectional views taken along the lines A-A′ and B-B′,respectively of FIG. 17A.

Next, the three kinds of gate insulating films used in the presentinvention are described.

First, it is desired that the second gate insulating film 14 used whencarriers are injected into the floating gate electrode 7 using tunnelingaccording to the present invention be a silicon oxide film formed bythermal oxidation that is excellent in film thickness controllabilityand film quality stability. Further, carriers are injected into thefloating gate electrode 7 only once or several times in the testing stepafter the semiconductor manufacturing process ends, and thus, specialconditions on the film formation and additional processing for thepurpose of obtaining high resistance to repeated erasing and writing,are not necessary. Meanwhile, the second gate insulating film 14 is setto have a thickness that is large enough that a desired tunnelingcurrent value is obtained when a drain voltage is applied that issufficiently higher than the operating voltage applied to thesemiconductor integrated circuit device in the testing step after thesemiconductor manufacturing process ends.

On the other hand, it is desired that the first gate insulating film 9and the third gate insulating film 15 have a large capacitance value.This is for the purpose of, when the drain voltage is applied to injectholes into the floating gate electrode 7 in the testing step,sufficiently lowering the potential of the floating gate electrode 7that is determined by capacitive coupling in order to efficiently applya voltage to the second gate insulating film 14.

An equivalent capacitive coupling circuit of the drain terminal 2 towhich a high potential is applied, the control gate electrode 8 and theP-type well region 5 that are set to have a low potential, and the bodyterminal 4 at the same potential as that of the P-type well region 5 isas illustrated in FIG. 18. As can be seen from FIG. 18, by setting thefirst gate insulating film 9 and the third gate insulating film 15 tohave a large capacitance and setting the ratio thereof to thecapacitance of the second gate insulating film 14 having a smallcapacitance and having a large thickness to be large, the potential ofthe floating gate electrode 7 at an intermediate potential of thecapacitive coupling is lowered. In this manner, a high voltage can beapplied to the second gate insulating film 14 formed between the drainterminal 2 and the floating gate electrode 7, and thus, the tunnelingcan be promoted.

Accordingly, the first gate insulating film 9 and the third gateinsulating film 15 are required to have a large electrode size when seenin a plan view. The reason is that the capacitance value can be large.It is sufficient that the first gate insulating film 9 and the thirdgate insulating film 15 have an area that is ten times or more as largeas the area of the second gate insulating film 14 when seen in a planview.

Further, for the purpose of having a large capacitance value, it isdesired that the first gate insulating film 9 and the third gateinsulating film 15 be as thin as possible. The floating gate electrode7, the control gate electrode 8, and the P-type well region 5 are fixedto the same low potential in terms of the circuit, and thus, theinsulating film thickness is not limited by the operating voltage of thesemiconductor integrated circuit device. Accordingly, in the case of athermal oxide film, taking into consideration leakage of carriers in thefloating gate electrode 7 due to a high temperature environment, it isdesired that the film thickness be from about 100 Å to about 200 Å.

Further, from the viewpoint of a large capacitance, it is desired thatthe first gate insulating film 9 and the third gate insulating film 15have a relative permittivity that is as high as possible, which can berealized by using SiON, SiN, HfO₂, or the like rather than a siliconthermal oxide film. Characteristics such as a threshold voltage of filmsother than a silicon thermal oxide film generally fluctuate to a largeextent due to instability of characteristics at a film interface.However, according to the present invention, even if there arevariations in characteristics, the variations are adjusted finally bythe regulation at the testing step, and thus, no problem arises.

Further, adopting the first gate insulating film 9 and the third gateinsulating film 15 that are thin or that have a high relativepermittivity has a strong point that the size thereof can be reduced andthe costs can be reduced accordingly. Further, this method leads toincrease in C as a capacitance value per unit area in the followinggeneral Expression (3), and thus, this method also has a strong pointthat the amount of fluctuations in threshold voltage can be lowered whenQ reduces due to leakage of carriers in the floating gate electrode 7 orthe like:

V=Q/C  (3)

As described above, by adopting the semiconductor nonvolatile memoryelement according to the present invention, the threshold voltage can beregulated with ease, the stability is high for a long time, andvariations in circuit characteristics due to variations in elementcharacteristics can be absorbed in electrical regulation in the testingstep, and thus, a highly accurate semiconductor integrated circuitdevice can be provided.

Next, a method of manufacturing the semiconductor nonvolatile memoryelement having the structure according to the first embodimentillustrated in FIG. 10 is described with reference to FIG. 20A to FIG.20D and FIG. 21E to FIG. 21H for illustrating process steps thereof.

First, the semiconductor substrate 1 of the P type or of the N type isprepared. After P-type impurities such as B or BF₂ are injected by ioninjection into a region in which the semiconductor nonvolatile memoryelement is to be formed, thermal diffusion is performed to form theP-type well region 5 (FIG. 20A).

The polarity of the semiconductor substrate 1 is selected in accordancewith a demand for the semiconductor integrated circuit device includingthe semiconductor nonvolatile memory element according to the presentinvention. Specifically, if the P-type well region 5 is required to beelectrically isolated so that the potential of the P-type well region 5is prevented from being the lowest potential on the semiconductorintegrated circuit device, it is desired that the N-type semiconductorsubstrate 1 be prepared. However, if the P-type well region 5 has thelowest potential on the semiconductor integrated circuit device, aP-type semiconductor substrate that is less expensive can be used.

The amount of impurities to be injected and conditions on the thermaldiffusion are selected so that the P-type well region 5 has an impurityconcentration of from 7×10¹⁵/cm³ to 7×10¹⁶/cm³ and has a depth of from 6μm to 10 μm. More specifically, the above-mentioned values are realizedby an area density of impurity injection of 1×10¹²/cm² to 1×10¹³/cm² andby performing the thermal diffusion through heat treatment at from1,100° C. to 1,200° C. for several hours to ten-odd hours.

Next, for the purpose of electrically isolating elements from oneanother, LOCOS or the like is used to form the element isolation region13 as a silicon oxide film around the P-type well region 5, and at thesame time, a semiconductor nonvolatile memory element region surroundedby the element isolation region 13 is defined (FIG. 20B).

Then, N-type impurities of As or P are injected by ion injection into aregion to be the drain region of the semiconductor nonvolatile memoryelement, to thereby form the N-type heavily doped impurity region 17 andthe first N-type lightly doped impurity region 18 (FIG. 20C).

The N-type heavily doped impurity region 17 serves as the lowerelectrode for injecting, using tunneling, carriers such as electrons orholes into the floating gate electrode 7 thereabove via the siliconoxide film in a later stage. In order to inhibit depletion on thesurface of the N-type heavily doped impurity region 17 when a highvoltage on the positive side is applied in the carrier injection, it isdesired that the N-type heavily doped impurity region 17 be formedthrough injection of As at an impurity concentration of 5×10¹⁸/cm³ ormore. The injecting energy in this case is to the extent that enablesthe impurities to pass through the oxide film on a surface of thesemiconductor substrate 1, and is, for example, about 100 keV.

Further, for the purpose of obtaining a drain breakdown voltage of adesired value or more, it is desired that the first N-type lightly dopedimpurity region 18 be formed by injecting P at an impurity concentrationof 1×10¹⁷/cm³ or more and 1×10¹⁸/cm³ or less. Further, by setting theinjecting energy to be 90 keV or more, the impurities can be diffused toa depth that is larger than that in the N-type heavily doped impurityregion 17, and a PN junction breakdown voltage with the P-type wellregion 5 below the N-type heavily doped impurity region 17 can be set tobe high.

Next, for the purpose of manufacturing the semiconductor nonvolatilememory element as a normally on type depression type MOSFET, N-typeimpurities of As or P are injected by ion injection into a region inwhich the channel is to be formed so that the threshold voltage may havea desired negative value, to thereby form the N-type channel impurityregion 10 (FIG. 20D).

Then, by thermal oxidation, CVD, or the like, the first gate insulatingfilm 9 having a thickness of from about 100 Å to about 200 Å is formedin the region in which the channel is to be formed, and the second gateinsulating film 14 having a thickness of about several hundred angstromsthat is larger than that of the first gate insulating film 9 is formedin a region in which the drain is to be formed (FIG. 21E).

The two gate insulating films having different thicknesses are realizedas follows. First, the thicker second gate insulating film 14 is formedover an entire surface of the element region as a silicon oxide film bythermal oxidation, then, the second gate insulating film 14 in regionsother than the region in which the drain is to be formed is removed byphotolithography and etching using HF or the like. After that, the firstgate insulating film 9 is formed as a silicon oxide film by thermaloxidation.

In this method, the second gate insulating film 14 is subjected tothermal oxidation when the first gate insulating film 9 is formed, andthe silicon oxide film forming the second gate insulating film 14 isgrown again. However, the second gate insulating film 14 already has alarge thickness, and thus, when the first gate insulating film 9 servingas a thin gate insulating film undergoes the thermal oxidation to beformed, the speed of oxygen reaching silicon is slow, and the filmthickness increases at a very low speed and the amount of the growth issmall. Accordingly, the thickness of the second gate insulating film 14after the thermal oxidation twice is dominantly affected by the firstthermal oxidation, and the thickness can be easily predicted.

Then, a polycrystalline silicon layer is formed on the first gateinsulating film 9 and the second gate insulating film 14, and impuritiesat a high concentration are injected by ion injection or thermaldiffusion so that the concentration is 1×10¹⁹/cm³ or more, and then,photolithography and dry etching are performed to form the floating gateelectrode 7 of the semiconductor nonvolatile memory element. At thistime, the floating gate electrode 7 and the second gate insulating film14 has an overlapping portion for the purpose of carrier injection usingtunneling (FIG. 21F).

Then, an insulating film is formed by thermal oxidation, CVD, or thelike on the floating gate electrode 7 of the semiconductor nonvolatilememory element for the purpose of forming the third gate insulating film15. After that, a polycrystalline silicon layer is formed in succession,and impurities at a high concentration are injected by ion injection orthermal diffusion so that the concentration is 1×10¹⁹/cm³ or more, andthen photolithography and dry etching are used to form the control gateelectrode 8 by patterning (FIG. 21G).

In this case, the floating gate electrode 7 and the control gateelectrode 8 may be collectively formed by performing photolithographyand dry etching once. Specifically, a polycrystalline silicon layer isformed on the first gate insulating film 9 and the second gateinsulating film 14, and impurities at a high concentration are injectedby ion injection or thermal diffusion so that the concentration is1×10¹⁹/cm³ or more. Then, the third gate insulating film 15 is formed insuccession by thermal oxidation, CVD, or the like. Further, apolycrystalline silicon layer is formed in succession, and impurities ata high concentration are injected by ion injection or thermal diffusionso that the concentration is 1×10¹⁹/cm³ or more. After that, the controlgate electrode 8 and the floating gate electrode 7 are collectivelyformed by patterning using photolithography and dry etching.

Then, in order to form the source/drain regions 12 of the semiconductornonvolatile memory element, N-type impurities of As or P are injected byion injection so that the concentration is 1×10²⁰/cm³ or more (FIG.21H).

The above description is made with reference to FIG. 20A to FIG. 20D andFIG. 21E to FIG. 21H for illustrating the process steps.

Then, an oxide film serving as an insulating film (not shown) is formedon an entire surface. After a contact hole is formed in a predeterminedlocation, for the purpose of giving the gate potential, the sourcepotential, the drain potential, and a body potential of thesemiconductor nonvolatile memory element, metal wiring is formed bysputtering and patterning a metal film.

Note that, when the structure of the fifth embodiment described withreference to FIG. 14A, FIG. 14B, and FIG. 14C in which the two-layergate electrode structure of polycrystalline silicon illustrated in FIG.10 is changed to the single-layer structure is manufactured, the processsteps up to the step of forming the floating gate electrode 7 on thefirst gate insulating film 9 and the second gate insulating film 14illustrated in FIG. 21F are common to the manufacturing method describedabove. After this, the source/drain regions 12 of the semiconductornonvolatile memory element are formed. In this way, the structure of thefifth embodiment can be similarly manufactured. The control gateelectrode 8 can be formed using, for example, the impurities and thestructure and the process step of the N-type heavily doped impurityregion 17 in common.

Next, a method of manufacturing the semiconductor nonvolatile memoryelement having the structure according to the second embodimentillustrated in FIG. 11 is described with reference to FIG. 22A to FIG.22D for illustrating process steps thereof. The second embodiment isdifferent from the first embodiment in that the second N-type lightlydoped impurity region 19 illustrated in FIG. 11 is additionally formed,and in FIG. 22A to FIG. 22D, process steps in a latter part aresimplified accordingly.

First, the semiconductor substrate 1 of the P type or of the N type isprepared. The P-type well region 5 is formed in a region in which thesemiconductor nonvolatile memory element is to be formed, and further,the second N-type lightly doped impurity region 19 is formed therein(FIG. 22A).

The amount of the impurities to be injected and conditions on thethermal diffusion of P-type impurities of B or BF₂ are selected so thatthe P-type well region 5 has an impurity concentration of from7×10¹⁵/cm³ to 7×10¹⁶/cm³ and has a depth of from 6 μm to 10 μm. Morespecifically, the above-mentioned value is realized by an area densityof impurity injection of from 1×10¹²/cm² to 1×10¹³/cm² and by performingthe thermal diffusion through heat treatment at from 1,100° C. to 1,200°C. for several hours to ten-odd hours.

Further, the amount of impurities to be injected and conditions on thethermal diffusion of N-type impurities of P or As are selected so thatthe second N-type lightly doped impurity region 19 has an impurityconcentration of from 2×10¹⁶/cm³ to 2×10¹⁷/cm³ and has a depth of from 3μm to 6 μm. The thermal diffusion may be performed when the heattreatment is performed in forming the P-type well region 5, or, may beperformed additionally thereafter.

Then, for the purpose of electrically isolating elements from oneanother, LOCOS or the like is used to form a silicon oxide film as theelement isolation region 13, and at the same time, the semiconductornonvolatile memory element region surrounded by the element isolationregion 13 is defined (not shown).

Then, N-type impurities of As or P are injected by ion injection into aregion to be the drain region of the semiconductor nonvolatile memoryelement to form the N-type heavily doped impurity region 17 and thefirst N-type lightly doped impurity region 18 (FIG. 22B).

It is desired that the N-type heavily doped impurity region 17 be formedby injecting As at an impurity concentration of 5×10¹⁸/cm³ or more, andthe injecting energy be about 100 keV so that the impurities can passthrough the oxide film on the surface of the semiconductor substrate.

Further, it is desired that the first N-type lightly doped impurityregion 18 be formed by injecting P at an impurity concentration of1×10¹⁷/cm³ or more and 1×10¹⁸/cm³ or less. By setting the injectingenergy to be 90 keV or more, the impurities can be diffused to a depththat is larger than that in the N-type heavily doped impurity region 17,and a PN junction breakdown voltage with the P-type well region 5 belowthe N-type heavily doped impurity region 17 can be set to be high.

After that, the N-type channel impurity region 10 is formed as describedabove with reference to FIG. 20D, and the first gate insulating film 9and the second gate insulating film 14 are formed as described abovewith reference to FIG. 21E.

Then, FIG. 22A to FIG. 22D are referred to again. A polycrystallinesilicon layer is formed on the first gate insulating film 9 and thesecond gate insulating film 14, and impurities at a high concentrationare injected by ion injection or thermal diffusion so that theconcentration is 1×10¹⁹/cm³ or more. Then, photolithography and dryetching are performed to form the floating gate electrode 7 of thesemiconductor nonvolatile memory element (FIG. 22C).

Then, the third gate insulating film 15 and the floating gate electrode7 are formed as described with reference to FIG. 21G.

Then, in order to form the source/drain regions 12 of the semiconductornonvolatile memory element, N-type impurities of As or P are injected byion injection so that the concentration is 1×10²⁰/cm³ or more (FIG.22D).

Then, an oxide film serving as an insulating film (not shown) is formedon an entire surface. After a contact hole is formed in a predeterminedlocation, for the purpose of giving the gate potential, the sourcepotential, the drain potential, and the body potential of thesemiconductor nonvolatile memory element, metal wiring is formed bysputtering and patterning a metal film.

Note that, when the structure of the sixth embodiment described withreference to FIG. 15A, FIG. 15B, and FIG. 15C in which the two-layergate electrode structure of polycrystalline silicon illustrated in FIG.11 is changed to the single-layer structure is manufactured, the processsteps up to the step of forming the floating gate electrode 7 on thefirst gate insulating film 9 and the second gate insulating film 14illustrated in FIG. 22C are common to the manufacturing method describedabove. After this, the source/drain regions 12 of the semiconductornonvolatile memory element are formed. In this way, the structure of thesixth embodiment can be similarly manufactured. The control gateelectrode 8 can be formed using, for example, the impurities and thestructure and the process step of the N-type heavily doped impurityregion 17 in common.

Next, a method of manufacturing the semiconductor nonvolatile memoryelement having the structure according to the third embodimentillustrated in FIG. 12 is described with reference to FIG. 23A to FIG.23D, and FIG. 24E and FIG. 24F for illustrating process steps thereof.

First, the semiconductor substrate 1 of the P type or of the N type isprepared. The P-type well region 5 is formed in a region in which thesemiconductor nonvolatile memory element is to be formed, and further,the second N-type lightly doped impurity region 19 and the first N-typelightly doped impurity region 18 are formed therein. The P-type wellregion 5 and the second N-type lightly doped impurity region 19 areformed as described in the manufacturing method for the secondembodiment. The first N-type lightly doped impurity region 18 is formedto have the impurity concentration of the N-type impurities of As or Pthat is adjusted to be from 1×10¹⁷/cm³ to 1×10¹⁸/cm³. The first N-typelightly doped impurity region 18 is formed in advance so as to underliethe thick oxide film 13 to be formed adjacent to the N-type channelimpurity region 10 (FIG. 23A).

Then, for the purpose of electrically isolating elements from oneanother, LOCOS is used to form the element isolation region 13, andthen, the thick oxide film 13 is formed on the first N-type lightlydoped impurity region 18. It is preferred that the thick oxide film 13formed on the first N-type lightly doped impurity region 18 have athickness of 1,000 Å or more. As described with reference to FIG. 12,the thick oxide film 13 may be formed simultaneously with the LOCOSoxide film 13 in the element isolation region to prevent increase in thenumber of process steps (FIG. 23B).

Then, N-type impurities of As or P are injected by ion injection into aregion to be the drain region of the semiconductor nonvolatile memoryelement to form the N-type heavily doped impurity region 17. Next, forthe purpose of manufacturing the semiconductor nonvolatile memoryelement as a normally on type depression type MOSFET, N-type impuritiesof As or P are injected by ion injection into a region in which thechannel is to be formed to form the N-type channel impurity region 10(FIG. 23C).

Then, by thermal oxidation, CVD, or the like, the second gate insulatingfilm 14 having a thickness that is larger than that of the first gateinsulating film 9 is formed in a part of the region in which the drainis to be formed on the N-type heavily doped impurity region 17 so as tobe in contact with the LOCOS oxide film 13 formed prior thereto. Then,the first gate insulating film 9 is formed on the region in which thechannel is to be formed. The two gate insulating films having differentthicknesses are realized as described with reference to FIG. 21E. First,the thicker second gate insulating film 14 is formed. Then, the secondgate insulating film 14 in regions other than the region in which thedrain is to be formed is removed by photolithography and etching usingHF or the like, and then, the first gate insulating film 9 is formed(FIG. 23D).

Then, a polycrystalline silicon layer is formed on the first gateinsulating film 9 and the second gate insulating film 14, and impuritiesat a high concentration are injected by ion injection or thermaldiffusion so that the concentration is 1×10¹⁹/cm³ or more. Then,photolithography and dry etching are performed to form the floating gateelectrode 7 of the semiconductor nonvolatile memory element. At thistime, the floating gate electrode 7 and the second gate insulating film14 has an overlapping portion for the purpose of carrier injection usingtunneling (FIG. 24E).

Then, the third gate insulating film 15 and the control gate electrode 8are formed as described with reference to FIG. 21G (not shown).

Then, in order to form the source/drain regions 12 of the semiconductornonvolatile memory element, N-type impurities of As or P are injected byion injection so that the concentration is 1×10²⁰/cm³ or more (FIG.24F).

The above description is made with reference to FIG. 23A to FIG. 23D,and FIG. 24E and FIG. 24F for illustrating the process steps.

Then, an oxide film serving as an insulating film (not shown) is formedon an entire surface. After a contact hole is formed in a predeterminedlocation, for the purpose of giving the gate potential, the sourcepotential, the drain potential, and the body potential of thesemiconductor nonvolatile memory element, metal wiring is formed bysputtering and patterning a metal film.

Note that, when the structure of the seventh embodiment described withreference to FIG. 16A, FIG. 16B, and FIG. 16C in which the two-layergate electrode structure of polycrystalline silicon illustrated in FIG.12 is changed to the single-layer structure is manufactured, the processsteps up to the step of forming the floating gate electrode 7 on thefirst gate insulating film 9 and the second gate insulating film 14illustrated in FIG. 24E are common to the manufacturing method describedabove. After this, the source/drain regions 12 of the semiconductornonvolatile memory element are formed. In this way, the structure of theseventh embodiment can be similarly manufactured. The control gateelectrode 8 cap be formed using, for example, the impurities and thestructure and the process step of the N-type heavily doped impurityregion 17 in common.

Next, a method of manufacturing the semiconductor nonvolatile memoryelement having the structure according to the fourth embodimentillustrated in FIG. 13 is described with reference to FIG. 25A to FIG.25D for illustrating process steps thereof.

First, the semiconductor substrate 1 of the P type or of the N type isprepared. The P-type lightly doped impurity region 20 and the secondN-type lightly doped impurity region 19 are formed in a region in whichthe semiconductor nonvolatile memory element is to be formed so as topartially overlap each other. The amount of impurities to be injectedand conditions on the thermal diffusion of N-type impurities of P or Asare selected so that the second N-type lightly doped impurity region 19has an impurity concentration of from 2×10¹⁶/cm³ to 2×10¹⁷/cm³ and has adepth of from 3 μm to 6 μm. The P-type lightly doped impurity region 20of B or BF₂ has, as described with reference to FIG. 13, an impurityconcentration of from about 2×10¹⁶/cm³ to about 2×10¹⁷/cm³, which is theimpurity concentration higher than that of the second N-type lightlydoped impurity region 19, thereby seeking improvement in the drainbreakdown voltage (FIG. 25A).

Then, the first N-type lightly doped impurity region 18 is formed in aregion in which the drain is to be formed to have the impurityconcentration of N-type impurities of As or P that is adjusted to befrom 1×10¹⁷/cm³ to 1×10¹⁸/cm³ (not shown).

Then, for the purpose of electrically isolating elements from oneanother, LOCOS is used to form the element isolation region 13, andthen, the thick oxide film 13 is formed on the first N-type lightlydoped impurity region 18. It is preferred that the thick oxide film 13formed on the first N-type lightly doped impurity region 18 have athickness of 1,000 Å or more. As described with reference to FIG. 12,the thick oxide film 13 may be formed simultaneously with the LOCOSoxide film 13 in the element isolation region to prevent increase in thenumber of process steps (FIG. 25B).

Then, the N-type channel impurity region 10, the first gate insulatingfilm 9, and the second gate insulating film 14 are formed (not shown).

Then, a polycrystalline silicon layer is formed on the first gateinsulating film 9 and the second gate insulating film 14, and impuritiesat a high concentration are injected by ion injection or thermaldiffusion so that the concentration is 1×10¹⁹/cm³ or more. Then,photolithography and dry etching are performed to form the floating gateelectrode 7 of the semiconductor nonvolatile memory element (FIG. 25C).

Then, the third gate insulating film 15 and the control gate electrode 8are formed (not shown).

Then, in order to form the source/drain regions 12 of the semiconductornonvolatile memory element, N-type impurities of As or P are injected byion injection so that the concentration is 1×10²⁰/cm³ or more (FIG.25D).

Then, an oxide film serving as an insulating film (not shown) is formedon an entire surface. After a contact hole is formed in a predeterminedlocation, for the purpose of giving the gate potential, the sourcepotential, the drain potential, and the body potential of thesemiconductor nonvolatile memory element, metal wiring is formed bysputtering and patterning a metal film.

Note that, when the structure of the eighth embodiment described withreference to FIG. 17A, FIG. 17B, and FIG. 17C in which the two-layergate electrode structure of polycrystalline silicon illustrated in FIG.13 is changed to the single-layer structure is manufactured, the processsteps up to the step of forming the floating gate electrode 7 on thefirst gate insulating film 9 and the second gate insulating film 14illustrated in FIG. 25C are common to the manufacturing method describedabove. After this, the source/drain regions 12 of the semiconductornonvolatile memory element are formed. In this way, the structure of theeighth embodiment can be similarly manufactured. The control gateelectrode 8 can be formed using, for example, the impurities and thestructure and the process step of the N-type heavily doped impurityregion 17 in common.

By the way, in the manufacturing methods of the first to fourthembodiments described above, the first insulating film 9 and the secondinsulating film 14 are formed in the same way. This is referred to as afirst manufacturing method of forming the gate insulating films. Insteadof the first method, the following second to fifth methods can beadopted for the purpose of improving the performance, reducing costs, orthe like.

The second method is different from the first method in that the threesteps of forming a thick oxide film, partly removing the oxide film, andforming a thin oxide film are not used, but the first gate insulatingfilm 9 and the second gate insulating film 14 having differentthicknesses are simultaneously formed by performing thermal oxidationonce.

Specifically, as illustrated in FIG. 26A, first, the N-type heavilydoped impurity region 17 is formed at a high impurity concentration of1×10¹⁹/cm³ or more using As.

Then, a gate insulating film is formed by wet oxidation using watervapor, or pyrogenic oxidation in which oxygen gas and hydrogen gas areintroduced into a furnace to be reacted with each other. The gateinsulating film has a large thickness only on the N-type heavily dopedimpurity region 17 due to an enhanced oxidation effect and has a smallthickness in a remaining region, and thus, a shape illustrated in FIG.26B is obtained.

In this case, for example, when the first gate insulating film 9 has athickness of 150 Å, the second gate insulating film 14 can have athickness of about 300 Å. As the extent of a lattice defect in thesemiconductor substrate 1 becomes larger, the effect of the enhancedoxidation becomes more remarkable due to entrance of oxygen andreaction. Thus, regardless of whether the impurities are of the N typeor of the P type, when the impurity concentration in the semiconductorsubstrate 1 is high, the effect of the enhanced oxidation can beobtained in accordance with the extent of the lattice defect. However,when the film is used as a gate insulating film in particular, an oxidefilm that is grown on an impurity region of the N type is desired.Accordingly, this method can be said to be effective in an N-channeltype semiconductor nonvolatile memory element. In this case, P-typeimpurities are not preferred because the P-type impurities enter theoxide film during the thermal oxidation to considerably lower thequality of the oxide film.

The method described above has effects that, by reducing the three stepsto one step, costs can be reduced and time taken for the step can beshortened.

Next, the third method of forming the first gate insulating film 9 andthe second gate insulating film 14 is described with reference to FIG.27A to FIG. 27C.

In the third method, first, a polycrystalline silicon layer 21A having athickness of from 100 Å to 400 Å is formed in advance on an entiresurface (FIG. 27A).

Then, the polycrystalline silicon layer 21A in regions other than aregion in which the second gate insulating film 14 is to be formed isremoved by photolithography and etching, and a polycrystalline siliconlayer 21B is left (FIG. 27B).

Then, the resultant is subjected to thermal oxidation for forming thefirst gate insulating film 9, and a silicon oxide film (9 and 14) isformed on the semiconductor substrate 1. At that time, by setting thesecond gate insulating film 14 to have a thickness so that thepolycrystalline silicon layer 21B is completely oxidized by the thermaloxidation for forming the first gate insulating film 9, the second gateinsulating film 14 may be formed of the oxide film formed by oxidizingpolycrystalline silicon. In this case, polycrystalline silicon is usedbecause, due to a lattice defect included therein, the oxidation speedcan be 1.5 to 2 times as high as that of ordinary monocrystallinesilicon (FIG. 27C).

The third method is different from the first method in that the need forlong-time high-temperature heat treatment for forming the thick secondgate insulating film 14 is eliminated, and thus, has effects thatvariations in impurities at a relatively low concentration in the N-typechannel impurity region 10, the first N-type lightly doped impurityregion 18, or the second N-type lightly doped impurity region 19 due toheat treatment at high temperature are inhibited and higher accuracy ofelement characteristics is promoted.

Next, the fourth method is described with reference to FIG. 28A to FIG.28C. First, an underlayer silicon oxide film having a thickness of from10 Å to 100 Å is formed in advance on an entire surface by thermaloxidation, and then, a SiN layer 22 having a thickness of from 100 Å to200 Å is formed on an entire upper surface of the underlayer siliconoxide film by LPCVD or the like (FIG. 28A).

Then, the SiN layer 22 in regions other than a region in which the firstgate insulating film 9 is to be formed is removed by photolithography(FIG. 28B).

Then, on the resultant silicon oxide film having a thickness of severalhundred angstroms for forming the second gate insulating film 14 isformed by thermal oxidation. At that time, the first gate insulatingfilm 9 is covered with SiN having a low reactivity, and thus, the oxidefilm is scarcely grown thereon. Accordingly, the first gate insulatingfilm 9 can be formed as a stacked film including the silicon oxide filmhaving a thickness of several ten angstromes and the SiN film having athickness of from 100 Å to 200 Å, and the second gate insulating film 14can be formed as a silicon film having a thickness of several hundredangstromes (FIG. 28C).

The fourth method has strong points that the first gate insulating film9 can have a large capacitance, the size of the gate electrode 8 can bereduced and costs can be reduced accordingly, and the amount ofvariations in threshold voltage can be small even when the charge Q isreduced due to leakage of carriers in the floating gate electrode 7, orthe like.

Next, the fifth method is described with reference to FIG. 29A to FIG.29D. First, similarly to the case of the first method, the second gateinsulating film 14 having a thickness of from 100 Å to 1,000 Å is formedon an entire surface as a silicon oxide film by thermal oxidation (FIG.29A).

Then, similarly to the case of the first method, the second gateinsulating film 14 in a region in which the channel is to be formed isremoved by photolithography and etching (FIG. 29B).

Then, the first gate insulating film 9 is formed by thermal oxidation.According to this method, the thickness of the first gate insulatingfilm 9 is smaller than that in the first method, and is from 30 Å to 100Å (FIG. 29C).

Then, thermal nitridation is performed in an atmosphere of ammonia at atemperature that is equal to or higher than 1,000° C. Then, nitrogen isdiffused to an interface with the semiconductor substrate 1 under thefirst gate insulating film 9 to react with the semiconductor substrate1, and a SiN layer having a thickness of from about 1 Å to about 20 Å isformed. On the other hand, the second gate insulating film 14 has asufficiently large thickness, and thus, the amount of nitrogen that isdiffused to reach the interface with the semiconductor substrate 1 isvery small. Thus, the extent of insulation of the formed SiN layer isnot large enough to prevent tunneling of the carriers (FIG. 29D).

The silicon oxide film that forms the first gate insulating film 9according to the fifth method has a small thickness of 100 Å or less,and thus, the carriers may dissipate in the floating gate electrode 7due to a leakage current at high temperature. However, the SiN layerunder the oxide film ensures a large extent of insulation, and thus, theleakage is inhibited, and at the same time, a large capacitance of thefirst gate insulating film 9 is realized as well.

The SiN layer is formed in the fourth method as well. However, when CVDis used as in the fourth method, controllability of a film thickness of100 Å or less is not satisfactory, and there is a problem in that theelement characteristics vary. Thermal nitridation as in the fifth methodcan form a thinner SiN layer with stability, which is effective for morehighly accurate element characteristics.

The present invention is applicable to products other than a step-downseries regulator and a voltage detector described above. Throughadoption of a memory terminal in which the threshold voltage can bechanged with an electrical signal that is input from an input terminalfor regulation, an output voltage can be changed with an inputelectrical signal in various semiconductor integrated circuit devicesincluding a reference voltage circuit. It goes without saying that thepresent invention is applicable to products other than a powermanagement IC.

What is claimed is:
 1. A semiconductor nonvolatile memory element,comprising: a semiconductor substrate; a well region of a firstconductivity type formed in the semiconductor substrate; a heavily dopedsource region and a first heavily doped drain region which are formed inthe well region so as to be apart from each other, and each of whichcontains high-concentration impurities of a second conductivity type; afirst gate insulating film formed on the semiconductor substrate betweenthe heavily doped source region and the first heavily doped drain regionso as to be adjacent to the heavily doped source region; a second gateinsulating film formed on the semiconductor substrate between theheavily doped source region and the first heavily doped drain region soas to be adjacent to the first heavily doped drain region; a secondheavily doped drain region of the second conductivity type formed apartfrom the heavily doped source region, the second heavily doped drainregion including a region under the second gate insulating film andbeing formed in a region that overlaps the first heavily doped drainregion; a first lightly doped drain region of the second conductivitytype formed apart from the heavily doped source region, the firstlightly doped drain region including the region under the first gateinsulating film and a region under the second gate insulating film andbeing formed in a region that overlaps the first heavily doped drainregion and the second heavily doped drain region; a channel impurityregion of the second conductivity type formed under the first gateinsulating film between the heavily doped source region and the firstlightly doped drain region; a floating gate electrode that is made ofpolycrystalline silicon containing high-concentration impurities, and isformed on the first gate insulating film and the second gate insulatingfilm; a control gate electrode formed in the well region formed at alocation apart from the channel impurity region, the control gateelectrode comprising a diffusion region containing high-concentrationimpurities of the second conductivity type; and a third gate insulatingfilm formed between the floating gate electrode that extends to abovethe diffusion region serving as the control gate electrode and thediffusion region serving as the control gate electrode, wherein thesecond gate insulating film has a thickness that is larger than athickness of the first gate insulating film, and wherein the well regionincludes the heavily doped source region, the first heavily doped drainregion, the second heavily doped drain region, the first lightly dopeddrain region, and the channel impurity region, and is formed to a depththat is larger than depths of the heavily doped source region, the firstheavily doped drain region, the second heavily doped drain region, thefirst lightly doped drain region, and the channel impurity region.
 2. Asemiconductor nonvolatile memory element according to claim 1, whereinthe impurities in the first heavily doped drain region comprise one ofAs and P at a concentration of 1×10²⁰/cm³ or more, wherein impurities inthe second heavily doped drain region comprise one of As and P at aconcentration of 5×10¹⁸/cm³ or more, wherein impurities in the firstlightly doped drain region comprise one of As and P at a concentrationof 1×10¹⁷/cm³ or more and 1×10¹⁸/cm³ or less, and wherein impurities inthe well region comprise boron at a concentration of from 7×10¹⁵/cm³ to7×10¹⁶/cm³.
 3. A semiconductor nonvolatile memory element according toclaim 1, wherein the first gate insulating film has a thickness of from100 Å to 200 Å.
 4. A semiconductor nonvolatile memory element accordingto claim 1, wherein the first gate insulating film comprises SiON andthe second gate insulating film comprises SiO₂.
 5. A semiconductornonvolatile memory element according to claim 1, wherein the first gateinsulating film comprises SiN and the second gate insulating filmcomprises SiO₂.